To raise concrete walls, formworks that define a volume in which concrete is poured are typically used. Usually, these formworks comprise: two walls placed face to face defining between them an interval where concrete is introduced, and connecting devices holding the walls with the required spacing. For example, connecting devices can be crossbars, whose ends support the formwork faces arranged one opposite the other and these crossbars being traversed by blocking pieces supported on the external faces of the formwork walls.
When the concrete is set, the formwork is recovered and the blocking pieces are withdrawn. The crossbars submerged in the concrete, which no longer have any function, can be left in place or withdrawn and do not contribute to in the strength of the wall, but on the contrary, make it more fragile.
It is essentially the quality of the concrete that confers the strength to the wall. To increase the strength of the wall with a classic formwork, horizontal and vertical reinforcements are introduced before the concrete is poured.
To increase the strength of a concrete wall, the use of permanent formworks whose water-proof or permeable walls constitute the skin of the wall is known. Mostly, these walls have waves that allow an anchorage of the skin in the concrete. This is known as collaborating formwork. The strength of the final composite structure built in this way is a superposition of the concrete strength and that of the formwork that constitutes the skin.
These walls are linked locally one to the other by crossbars traversed by recoverable blocking pieces or by fastenings whose ends anchor on said formwork walls. These fastenings extend in essentially an orthogonal direction to the formwork faces. In this case, formwork walls and the fastenings that link them help in the reinforcement of the structure of the concrete wall. According to the type of fastenings, a thermal and a mechanic decoupling for both formwork walls can be obtained.
A formwork of the type described above is discussed in the patent FR-A-2 675 181. The articulated connecting elements allow the reduction of the number of assembly operations, and facilitate the setting of the formwork while keeping its strength and its conformity to security and manufacturing standards, and have a lightweight structure.
The document WO 97/31165 describes an improvement using a slender element bent in a zigzag shape as an articulated connecting element, which distributes the forces in the concrete, parallel to the external faces of the wall, so that a skin having a hoop effect is formed. Thus, the wall is reinforced.
However, with this known device, it seems necessary or desirable to include an internal reinforcement, which is difficult to make while the formwork is being manufactured. Thus, it is necessary, on the construction site, to insert manually separated stiffeners inside the formwork. This operation is expensive and time consuming.